The selective delivery of therapeutic agents to diseased tissue, in vivo, remains a major challenge in the interests of improved therapeutic outcomes. It must be appreciated that although much of the following discussion of the invention is specified toward anti-cancer treatments, any disease state amenable to treatment with drugs or prodrugs could be addressed in the same way. In the case of cancers, standard chemotherapies have depended, in general, upon the enhanced uptake of toxic drugs by fast-growing diseased cells, in relation to most normal cells. However, this has been found to be only of limited value, and normal cell toxicities are often reached before a decisive therapeutic effect against the cancer can be obtained. Typically, those normal cells that divide the fastest are most prone to the adverse effects of chemotherapy agents.
Several different approaches are being taken that seek to improve the therapeutic outcomes resulting from anti-cancer drug therapies. One is the use of mixtures or ‘cocktails’ of drugs, with component drugs often chosen for their effects on different aspects of cell metabolism. A second is the encapsulation of drugs into carriers such as liposomes or the attachment of drugs to long-circulating polymers. This approach extends drug half-life in serum, and generally allows for a greater proportion of the administered drug to be deposited at the target site. A third approach can be viewed as an advance on the second approach, in that drugs are attached to specific targeting agents such as monoclonal antibodies or peptides. These agents are able to specifically accrete at a target due to their binding to an antigen or receptor, respectively, which has been upregulated or specifically produced by the target cells.
A disadvantage with the aforementioned approaches is the tendency of drugs to lose potency upon conjugation to a polymer, peptide or monoclonal antibody (MAb). Numerous articles have described methods of drug conjugation that seek to preserve drug activity while forming a stable bio-conjugate. Unfortunately many drug-carrier conjugates also dissociate when subjected to the challenge of an in vivo serum environment. Moreover, tumor uptake of the drug is often reduced while non-specific toxicity to normal tissues is often increased.
An advanced method for delivering a drug to a disease site in a less toxic and more efficient and efficacious manner, is the use of antibody-directed enzyme prodrug therapy (ADEPT). See, for example, U.S. Pat. No. 5,632,990 (Bagshawe). Originally, ADEPT depended on the use of a conjugate of an antibody and an enzyme to localize the latter to a site of disease. Such an approach had several drawbacks, including loss of antibody and/or enzyme activity upon conjugation, and high residual levels of circulating MAb enzyme conjugate in the bloodstream due to the long-circulating MAb. The latter, in turn, resulted in excessive enzyme activity in circulation upon administration of the prodrug, which was cleaved by enzyme in the bloodstream, generating high levels of active drug, and high levels of non-specific drug toxicity.
Later, the use of bispecific antibodies (bsAbs) was suggested for application to the ADEPT method. In this approach, a bispecific antibody targeting both a disease-associated antigen with one arm, and an epitope on an enzyme with a second arm would be given to a subject, followed some time later by the enzyme in question, and finally by the prodrug that the enzyme was active against. This comprises a three-step delivery system, absent any clearing agents. Difficulties encountered in the practice of this ADEPT method in the second capture step, that is by the second arm of the bsAb against the enzyme epitope, perhaps due to low affinity of this antibody-antigen complex, led to protocols where the bsAb and the enzyme were mixed together, and administered as a single complex, followed later by the prodrug. This altered approach comprises a two-step delivery system, absent any clearing agents. However, this modified ADEPT method remained fraught with problems preventing its ultimate wide application in patients. These included the utility of the targeting arm of the bsAb, bsAb preparation issues, binding affinity of the second (anti-enzyme epitope) arm of the bsAb, choice of prodrug, efficiency of prodrug cleavage by the enzyme, and, not least, presence of active enzyme in non-target tissues at the time of prodrug administration. The latter leads to unwanted cleavage of prodrug in normal tissues, and, subsequently, untoward toxicity due to the generation of active drug in those tissues. A particular problem was encountered in the cleavage of prodrug to drug in the circulation by active enzyme.
A continuing need therefore exists for methods and compositions that are able to selectively deliver therapy agents to a disease site using an ADEPT approach, without undue dissociation of bsAb and enzyme, and without adversely affecting a therapeutic agent's potency.